Each time the clutch is engaged, the friction liners become progressively gripped between a pressure plate and a reaction plate of the clutch. As is shown in the specification of U.S. Pat. No. 2,902,130, the heating which is caused by the corresponding friction effects leads inevitably to the pressure plate eventually adopting a conical shape. The same is true of the reaction plate, albeit to a lesser extent. The result is that the zone in which the friction liners are subjected to pressure from one or other of the pressure and reaction plates are displaced progressively towards the axis of the assembly. A consequence of this is uneven wear of the friction liners, with a reduction in their effectiveness and possibly even damage to the liners.